Process for forming a multilayer coextruded article and articles therefrom

ABSTRACT

A process for coextrusion-molding a multilayer article comprising coextruding at a selected coextuding temperature (i) a first outer polymer resin layer having a) a viscosity, b) a melting temperature, and c) a degradation temperature at the selected coextuding temperature, and (ii) a second inner polymer resin layer having a) a viscosity, b) a melting temperature, and c) a degradation temperature at the selected coextuding temperature, wherein the ratio of the outer polymer resin viscosity to the inner polymer resin viscosity at the coextuding temperature is less than or equal to about 1 and the coextuding temperature is above the melting temperature of the highest melting resin and below the degradation temperature of the lowest degrading resin to form a multilayer article.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional of, and claims the benefit of,application Ser. No. 09/516,311, filed Mar. 1, 2000, which status isallowed. The Ser. No. 09/516,311 application is a continuation-in-partapplication of U.S. Ser. No. 09/378,262, filed Aug. 20, 1999, nowabandoned, which claims priority to provisional patent applicationSerial No. 60/097,246, filed Aug. 20, 1998. U.S. application Ser. Nos.09/516,311, 09/378,262, and 60/097,246 are each incorporated herein byreference in their entireties.

FIELD OF THE INVENTION

[0002] This invention relates generally to a process comprisingcoinjecting or coextruding a structural polymer resin with one or moreperformance polymer resins to a form a multilayer article without meltflow defects.

BACKGROUND OF THE INVENTION

[0003] Poly(ethylene terephthalate) (PET) is an established bottlepolymer that produces rigid bottles with excellent clarity and gloss.These containers are manufactured by a process that comprises drying thePET resin, injection molding a preform and, finally, stretch blowmolding the finished bottle.

[0004] The injection molding of PET preforms requires the melting ofpolymer pellets and the injection of the molten, viscous PET materialinto a cavity, which also has a core rod. The molten PET forms a “skin”where it comes into contact with the cold cavity wall and core rod. Thisskin is composed of “frozen” PET and will remain fairly stationarythroughout the remainder of the injection molding process.

[0005] At points extending radially inwardly away from the cavity walland, outwardly from the core rod, or at the points at which the polymerdoes not directly contact the cavity wall or core rod, the polymer(which is still elevated in temperature) remains a viscous, flowingmass. This hot inner viscous material can still flow relative to thefrozen skin layer although its viscosity increases as it continues tocool. Thus, a temperature transition region occurs in the radialdirection as well as a corresponding melt viscosity transition (becauseof PET's viscosity dependence upon temperature). Regardless of thechanges in melt viscosity as a function of radial distance from theskin, monolayer PET is, for the most part, unaffected by the shear thatdevelops between the frozen skin of the PET and the molten polymer thatpushes past it. After the entire cavity has been filled using thisprocess, the polymer is held in the cavity until the preform has becomesufficiently cool so that it can be blown immediately into a bottle orthe preform is cool enough to be ejected. Cooled preforms that have beenejected are stored for later reheat blow molding into the final product.

[0006] Using this process, PET resin is used in a wide range ofapplications such as carbonated soft drink, hot-filled juice productsand warm-filled foods. However, PET has insufficient barrier to meet thedesired shelf lives of products with more demanding gas barrier needs.

[0007] In one particular application, in order to increase the gasbarrier of a PET bottle, it is possible to inject a barrier layer intoor onto a preform during the injection molding process. This barrierlayer is injected into or onto the melt flow stream of the PET such thatthe barrier polymer resin flows past the skin of PET previouslyinjected. This “coinjection” process allows two resins to be injectedinto a “multilayer” preform that can be blown to form the final bottleproduct.

[0008] Unfortunately, it has been found that the coinjection of abarrier polymer resin with PET can result in defects in the PET preform.A commonly observed melt flow defect is small “pulls,” frequently calledchevrons because of their V shape. Chevrons are interfacialinstabilities that occur between layers. Chevrons detract from theaesthetics of the finished article.

[0009] One barrier resin that may be used in a multilayer process is anethylene-vinyl acetate copolymer (EVOH) modified with various levels ofethylene (“grades”). It is commonly known that these “grades” of barrierresins have different melt viscosities and melting points. Generally, itwould be desirable to match both the melt viscosity of the barrier resinand the melt temperature of the barrier resin to the PET being used.Unfortunately, the commercially available EVOH (regardless of the grade)has a melt viscosity and degradation temperature far below that ofcommercially available PET. In addition, heat transfer from the hotterPET layer will further heat the EVOH above its desired processingtemperature and result in even lower melt viscosity of the barrier resinduring injection molding.

[0010] Most of the technology for coinjection is relatively new and isjust becoming commercially viable for molding multilayer articles orpreforms on a large scale. In addition, coinjection for most practicalpurposes is focused almost solely on the use of PET (or a copolymerthereof) as the structural resin for preform molding applications. Incontrast, coextrusion is a well-established technique that is commonlyapplied to a wide variety of different polymers (e.g., PET,copolyesters, polyolefins, PVC, styrenics, nylons, etc.) and for a muchwider range of applications.

[0011] In coextrusion, multilayer film or sheet is produced as opposedto a molded article. As with coinjection, there is one or more“structural” layers combined with one or more “performance” layers. Thestructural layers are usually (but not always) cheaper than theperformance layers and are included to keep total cost down (sinceperformance layers can often be expensive). Examples of coextrusioninclude the use of a barrier layer in packaging film, the use of a UVprotecting layer on the outside layer of heavy gauge sheeting foroutdoor weathering protection, the use of regrind in the center toreduce costs, the use of adhesive/sealing layers on the outside surface,and the use of glossy and/or pigmented layers to change the overallaesthetics of the film/sheet. Unlike the coinjection example citedabove, the “performance” layer in coextrusion does not necessarily haveto be on the inside of the multilayer structure.

[0012] In the process of coextrusion, the various resins are firstmelted in separate extruders and then brought together in a feedblock-afeedblock being nothing more than a series of flow channels which bringthe layers together into a uniform stream. From this feedblock, thismultilayer material then flows through an adapter and out a film die.The film die can be a traditional flat film/sheet die (e.g., acoathanger die) or it can be an annular die as is used in blown film.Coextrusion is also used making more complicated shapes like profiles.When we refer to coextrusion in this document, it is implied that all ofthese other coextrusion applications are also covered in addition totraditional film/sheet applications.

[0013] As with coinjection, coextrusion often suffers with the problemof chevrons and other visual defects. These defects in coextrusion andcoinjection both result from high shear stresses developing at the layerinterface during flow. These stresses are a function of the viscositiesof the layers in addition to the relative position and thickness of thelayers. In fact, knowledge gained from coextrusion can be used to helpminimize the flow defects in coinjection.

[0014] In addition, coextrusion of flat film often suffers from theproblem of poor layer distribution across the width of the sheet. Forexample, if one were to take a piece of coextruded film (for example, anA/B/A structure) and separate the layers, they might find that one ofthe A layers would be much thicker near the outer edges of the sheet,and very thin in the middle. The B layer would be just the opposite,that is, being thin near the edges and thick in the middle. Usually, itis desired that the layers be uniform in thickness across the full widthof the sheet so that properties (e.g., barrier, color, stiffness, etc.)do not vary across the width.

[0015] Up until now, correcting these two coextrusion problems (poorlayer distribution uniformity and flow defects) has really been more ofan art than science. There have been some attempts to balance theviscosities of the resins (i.e., having a viscosity ratio close to one)to improve layer distribution, but this has met with only limitedsuccess. Thus, there exists a need for a process to properly select boththe resin viscosity and elasticity parameters and the processingconditions in coextrusion such that both the interfacial instabilities(i.e., visual defects like chevrons) and poor layer distribution areeliminated.

[0016] In the coextrusion process according to this invention,therefore, the “elasticity” of the various resin layers is as importantas the resin viscosity and proper balancing of both the elasticity ratioand the viscosity ratio simultaneously is needed in order to have auniform layer distribution and form a multilayer article. A process hasthus been developed so that processing conditions and resins can beoptimized to eliminate these multilayer flow problems.

[0017] Because the multilayer flow behavior is very similar for bothcoinjection and coextrusion, the method can be effectively applied forboth applications. As a result, the process of the present inventionforms a high quality coinjected multilayer article or preform as easilyas it forms a multilayer coextruded film structure.

SUMMARY OF THE INVENTION

[0018] The present invention relates to the elimination of melt flowdefects such as chevrons from coinjected and/or coextruded articles byminimizing the interfacial stress between layers, such as between astructural layer (e.g., PET) and a performance (e.g., barrier) layer, ina multilayer molded structure or article.

[0019] In addition, the present invention relates to the matching ofviscoelastic flow properties of the respective layers so that layerdistribution is maintained in a uniform fashion for coextrusion andcoinjection applications.

[0020] As embodied and broadly described herein, this invention, in oneembodiment, relates to a process for coinjection-molding a multilayerarticle. The process comprises coinjecting at a selected coinjectingtemperature (i) a first outer polymer resin layer having a viscosity atthe selected coinjecting temperature, and (ii) a second inner polymerresin layer having a viscosity at the selected coinjecting temperature,wherein the ratio of the outer polymer resin viscosity to the innerpolymer resin viscosity at the coinjecting temperature is less than orequal to about 2 and the coinjecting temperature is above the meltingtemperature of the highest melting resin and below the degradationtemperature of the lowest degrading resin to form a multilayer article.

[0021] In another embodiment, the present invention comprises a processfor coextruding a multilayer article comprising coextruding at aselected coextruding temperature (i) a first outer polymer resin layerhaving a viscosity at the selected coextruding temperature, and (ii) asecond inner polymer resin layer having a viscosity at the selectedcoextruding temperature, wherein the ratio of the outer polymer resinviscosity to the inner polymer resin viscosity at the coextrudingtemperature is less than or equal to about 2 and the coextrudingtemperature is above the melting temperature of the highest meltingresin and below the degradation temperature of the lowest degradingresin.

[0022] In another embodiment, the present invention relates to a processfor coinjection-molding a 5-layer article comprising coinjection-moldingat a selected coinjecting temperature (i) two outer polymer resin layershaving a viscosity at the selected coinjecting temperature, (ii) twointermediate polymer resin layers disposed between a core layer and thetwo outer layers, the two intermediate resin layers having a viscosityat the selected coinjecting temperature, and (iii) a core layer having aviscosity at the selected coinjecting temperature, wherein at eachpolymer resin interface the ratio of the outermost polymer resinviscosity to the next innermost polymer resin viscosity at thecoinjecting temperature is less than or equal to about 2 and thecoinjecting temperature is above the melting temperature of the highestmelting resin and below the degradation temperature of the lowestdegrading resin to form a 5-layer article.

[0023] In yet another embodiment, the present invention relates to aprocess for coextruding a 5-layer article comprising coextruding at aselected coextruding temperature (i) two outer polymer resin layershaving a viscosity at the selected coextruding temperature, (ii) twointermediate polymer resin layers disposed between a core layer and thetwo outer layers, the two intermediate resin layers having a viscosityat the selected coextruding temperature, and (iii) a core layer having aviscosity at the selected coextruding temperature, wherein at eachpolymer resin interface the ratio of the outermost polymer resinviscosity to the next innermost polymer resin viscosity at thecoextruding temperature is less than or equal to about 2 and thecoextruding temperature is above the melting temperature of the highestmelting resin and below the degradation temperature of the lowestdegrading resin to form a 5-layer article.

[0024] Additional advantages of the invention will be set forth in partin the detailed description, including the figures, which follows, andin part will be obvious from the description, or may be learned bypractice of the invention. The advantages of the invention will berealized and attained by means of the elements and combinationsparticularly pointed out in the appended claims. It is to be understoodthat both the foregoing general description and the following detaileddescription are exemplary and explanatory of preferred embodiments ofthe invention, and are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE FIGURES

[0025]FIG. 1 is a plot of a frequency sweep to determine dynamicviscosity data for EVOH at 180EC.

[0026]FIG. 2 is a schematic diagram of velocity and shear stressprofiles in a polymer resin flow channel.

[0027]FIG. 3 is a plot of λ(A)/λ(B) versus η(A)/η(B) illustrating theoptimum operating region for eliminating chevrons and balancing layerthickness.

[0028]FIG. 4 is a plot of λ(A)/λ(B) versus η(A)/η(B) for an A/B/Acoextrusion of a PE (polyethylene) cap layer onto a PETG (polyethyleneterephthalate-G) core layer. The points refer to the differentcombinations of processing temperatures as outlined in Example 1.

[0029]FIG. 5 is a plot of λ(A)/λ(B) versus η(A)/η(B) for an A/B/Acoextrusion of a PE (polyethylene) cap layer onto a PETG (polyethyleneterephthalate-G) core layer. Extrusion temperatures for both layers wereheld constant at 235EC and the PE melt index varied from 0.9 to 3.2.

[0030]FIG. 6 is a plot of λ(A)/λ(B) versus η(A)/η(B) for an A/B/Acoextrusion of a PC (polycarbonate) cap layer onto a PET (polyethyleneterephthalate) core layer. Extrusion temperatures for both layers weredifferent.

DETAILED DESCRIPTION OF THE INVENTION

[0031] The present invention may be understood more readily by referenceto the following detailed description of the invention, including theappended figures referred to herein, and the examples provided therein.It is to be understood that this invention is not limited to thespecific processes and conditions described, as specific processesand/or process conditions for processing molded articles as such may, ofcourse, vary. It is also to be understood that the terminology usedherein is for the purpose of describing particular embodiments only andis not intended to be limiting.

[0032] It must also be noted that, as used in the specification and theappended claims, the singular forms “a,” “an” and “the” comprise pluralreferents unless the context clearly dictates otherwise. For example,reference to processing or forming an “article,” “container” or “bottle”from the process of this invention is intended to comprise theprocessing of a plurality of articles, containers or bottles.

[0033] Ranges may be expressed herein as from “about” or “approximately”one particular value and/or to “about” or “approximately” anotherparticular value. When such a range is expressed, another embodimentcomprises from the one particular value and/or to the other particularvalue. Similarly, when values are expressed as approximations, by use ofthe antecedent “about,” it will be understood that the particular valueforms another embodiment.

[0034] Overview

[0035] The present invention relates to the elimination of melt flowdefects such as chevrons from coinjected and/or coextruded articles byminimizing the interfacial stress between layers, such as between astructural layer (e.g., PET) and a performance (e.g., barrier) layer, ina multilayer molded structure or article. Preferably, these melt flowdefects may be minimized while obtaining and retaining the preferredphysical properties in the resulting article.

[0036] In addition, the present invention relates to the matching ofviscoelastic flow properties (i.e., both the viscosity and elasticityproperties) of the respective layers so that layer distribution ismaintained in a uniform fashion for coextrusion and coinjectionapplications.

[0037] In one embodiment, it is possible to inject a barrier layer intoa preform during the injection-molding process. The barrier layer isinjected into the melt flow stream of PET (structural layer) such thatthe barrier resin flows past the skin (preferably on the inside) of thePET previously injected. Preferable barrier resins used in themultilayer process include EVOH modified with various levels of ethylene(“grades”) and poly(m-xylylene adipamide) (MXD6). This “coinjection”process allows two resins to be injected into a “multilayer” preformthat can be blown to form the final bottle product. However, it iscommonly known that these “grades” of barrier resins have different meltviscosities and melting points. When wide differences in melt viscosityoccur between PET and the barrier resin, the potential for formation ofvisible melt flow defects increases.

[0038] In addition, the viscosity ratio of the two polymers also playsan important role in the formation of visible defects. One example of avisible flow defect is a v-shaped chevron. Chevrons are interfacialinstabilities that occur between layers when the shear stress at theinterface exceeds a critical value. It is, therefore, desirable to matchthe melt viscosity of the barrier resin with the PET resin to eliminatevisible defects, particularly chevrons.

[0039] To eliminate the visible defects, it is necessary to develop aset of processing conditions and carefully select a grade of PET orother structural polymer resin such that the melt viscosity andprocessing temperatures can be matched as closely as possible to theperformance or barrier polymer resin. By more closely matching theseparameters, the shear stress at the interface between layers can bereduced thereby producing a higher quality, more resilient anddefect-free multilayer container. For example, a CHDM-modified PET withdramatically reduced I.V. may be selected so that it can be processed insuch a manner that preforms and the resulting bottles can be producedwithout visual defects.

[0040] One embodiment of the present invention comprises minimizing thestress by balancing the structural polymer layer/performance polymerlayer viscosity ratio in the coinjection and coextrusion processes. Forexample, the ratio of the viscosity of the structural polymer resindivided by the viscosity of the performance polymer resin is preferablyless than or equal to about 2, and more preferably less than or equal toabout 1 to form a multilayer shaped article without melt flow defects.Most preferably, the ratio of the structural resin melt viscosity to theperformance resin melt viscosity is less than or equal to about 1 andgreater than or equal to about 0.5. Widely differing viscosity ratioscan lead to distortion of the shape of the interface as well as highinterfacial stresses (the latter causing chevrons).

[0041] In the coextrusion of films and sheets, interfacial instabilitiesare more likely when the outer (cap) layer is very thin, typically lessthan 10% of the total thickness because the shear stresses duringcoextrusion are highest near the outer surface. To overcome this, and tominimize the stress and eliminate chevrons, the outer (cap) layer mayhave a slightly lower viscosity than the inner (core) layer. However,the viscosity ratio is still close to one. However, for a coinjectionprocess, the situation is reversed. The outer layer(s) preferablycomprises at least one suitable structural polymer resin with a verythin inner (core) layer(s) of a performance polymer resin. Whilestresses are naturally much lower in the middle of the flow channelwhere, typically, the performance resin layer flows, it is stillpossible for them to become high enough for chevrons to occur.

[0042] For purposes of this invention, when forming a multilayer articleby a coinjection or coextrusion process, the outer or outermost resinlayer of a multilayer material is defined as the layer that is closestto the surface of the metal wall(s), which define the flow channel. Themetal wall(s) may typically include the surfaces of the mold wall andcore pin, for example. Each subsequent layer toward the center line ofthe flow channel is considered an inner layer relative to the layercloser to the wall(s). In other words, for every interface of two ormore layers, an outer layer is defined as the closest to the surface ofany wall.

[0043] In contrast to coextrusion, it is preferable in coinjection ifthe inner preferably performance polymer resin has a viscosity that isequal to or slightly higher than the viscosity of the structural polymerresin (with the viscosity ratio still being within the above-definedranges). Currently, for example, the viscosity of EVOH is much less thanthat of PET at the processing temperatures used, which greatlycontributes to the formation of chevrons.

[0044] Both viscosity and elasticity will vary with temperature.Therefore, it is important to select processing conditions which (a)most closely match the optimum viscosity and elasticity temperatures ofthe selected resins and (b) are within known resin constraints, such asthe melting point and degradation temperature for each of the selectedpolymers. In fact, according to the processes of this invention, thecoinjecting or coextruding temperature should be above the meltingtemperature of the highest melting polymer resin used, which may or maynot be the structural or performance polymer resin. Further, thecoinjecting or coextruding temperature should be below degradationtemperature of the lowest degrading polymer resin, which may or may notbe the structural or performance polymer resin.

[0045] It should be appreciated that for many structural/performanceresin combinations there will be more than one set of suitableprocessing temperatures within the ranges of the present invention. Thisis highly desirable as it allows for optimization on varying molding andextruding equipment. Thus, it is desirable to choose a set of optimumelasticity and viscosity temperatures that are within the polymer resinset constraints and are reasonably close together or matched, andprovide an elasticity and viscosity ratio within the range of thepresent invention.

[0046] Viscoelastic Parameters

[0047] In order to apply the processes described herein, it is firstnecessary to define the viscoelastic properties of each layer as afunction of temperature. The viscoelastic properties are viscosity andelasticity. Both of these parameters decrease with increasingtemperature and at varying rates depending on the type of polymer(polymer I.V., M_(w), etc.).

[0048] The viscosity is simply the ratio of the shear force exerted by afluid divided by the applied shear rate. More viscous fluids like honeyor oil have a greater resistance to flow than less viscous fluids likewater. In contrast, “elasticity” is a measure of the “memory” or“rubberiness” of a fluid. A highly elastic fluid, after deformed, willtry to return to its original undeformed shape once the stress isremoved. A rubber band is an extreme example of a highly elasticmaterial. In contrast, a material with no elasticity (e.g., a purelyviscous fluid like water) will have no memory of its original shape andwill not try to “snap back” to its original shape after the stress isremoved.

[0049] Polymers fall in between the two extremes of purely elastic(e.g., a rubber band) and purely viscous, with the degree of elasticitydepending on such things as the molecular weight, degree of chainentanglement, etc. Polymers also have what is known as a “fadingmemory”. In other words, the polymer's memory of a stress event willgradually decrease with increasing time. So if one applies a stress to apolymer with a fading memory and waits a long time before releasing, thepolymer will have little or no “snap back” because its memory of theevent is gone. The time that it takes for most of the memory to fade iscalled the “relaxation time” and is denoted as λ. For purposes of thisinvention, relaxation time is used as a measure of “elasticity.”

[0050] Elasticity is important in coextrusion or coinjection processingin particular because every time a polymer undergoes a change in flowcondition (which results in a change in stress), the polymer retainssome memory of that stress which affects its flow further downstream.For example, the hot runners that connect the extruder and injectionmold often have a 90 degree elbow in the piping to change the directionof the flow. This elbow also imparts a different stress to the polymeras it goes around the bend. If the time it takes for the polymer to flowfrom the bend to the gate is less than the relaxation time λ, then thestresses in the bend will still be remembered by the resin whichconsequently can affect the flow in the mold. In some cases, this canlead to the polymer preferentially and non-symmetrically filling up oneside of the preform mold. This affects cooling behavior and possiblyeven layer distributions for multilayer preforms. For coextrusion, theimparted stresses occur when the layers are first brought together inthe feedblock and continue at different points as the polymer flowsthrough the adapter and then into the die. Because of the differences inelasticity between the different layers in a multilayer flow, theelastically induced stresses at the interface will cause gradualrearrangement of the interface as it flows down the channel.

[0051] Therefore, the longer the relaxation time, the more rubbery (asopposed to liquid-like) the polymer will behave. The elasticity ratioshould fall roughly within the same range as the viscosity ratio.However, it is preferred to have the elasticity ratio be approximatelyor equal to the reciprocal of the viscosity ratio to offset or balancethe tendency of the mismatched polymer viscosities, which may createnon-uniform layers. One method of quantifying the reciprocal value andfor defining a range that is acceptable in the processes of thisinvention is to require that

[0052] −0.2<log 10(elasticity ratio)+log 10(viscosity ratio)<0.2

[0053] In this range, the variation in layer thickness is less thanabout 25% from center to outer edge for a coextruded structure. In theabove equation, if the elasticity ratio is the exact reciprocal of theviscosity ratio, then the above term equals 0. However, this value mayrange from −0.2 to 0.2 with a more preferred range from −0.1 to 0.1,which gives about +/−10% on the thickness variation.

[0054] Thus, in one embodiment of the present invention, the ratio ofthe elasticity of the structural polymer resin to the elasticity of theperformance polymer resin is preferably between 0.5 and 2, and morepreferably equal to or slightly greater than 1, the more preferred casestemming from the fact that the preferred viscosity ratio is slightlyless than 1. As the elasticity ratio begins to deviate excessively fromthe reciprocal of the viscosity ratio, layer thickness non-uniformitywill become severe. Also, the die swell of each polymer as it exits theinjector nozzle and enters the mold is a strong function of itsrelaxation time. Therefore, a balanced elasticity ratio between thestructural (e.g., PET) and performance (e.g., EVOH barrier) resins willlead to similar die swells that improve flow and layer uniformity.

[0055] The melt viscosity, elasticity, and processing temperatures forcommercially available EVOH barrier resins, for example, have beenpredetermined. It is, therefore, desirable to develop a set ofprocessing conditions and carefully select a grade of PET such that themelt viscosity, elasticity, and processing temperatures can be matchedas closely as possible to the performance resin in order to eliminateflow defects (such as chevrons) from occurring and to keep the layerthickness distribution uniform. Thus, the shear stress at the interfacebetween layers can be reduced producing a higher quality, more resilientand defect-free multilayer article or container.

[0056] Polymer melt viscosities are known to be proportional to M_(w)^(3.4), where M_(w) ^(3.4) is the weight average molecular weight.Because M_(w) is directly related to I.V., melt viscosity is thereforedirectly proportional to I.V^(5.1), where I.V.^(5.1) is measured in60/40 phenol/tetrachloroethane at 25° C. Thus, by reducing the I.V. ofthe PET resin, for example, the melt viscosity can also be decreased.

[0057] Polymer relaxation times will also decrease with decreasing I.V.,so it is important to try to balance both the viscosity and elasticityratios at the same time. Usually this involves a tradeoff in that bothratios may not be made to exactly equal 1. Thus, for coextrusion, it isnot necessary for both the viscosity and elasticity ratios to be thesame, but it is preferable if the two ratios offset one another (theelasticity ratio should be the reciprocal to the viscosity ratio). Inother words, if the elasticity ratio is slightly greater than one, thenthe viscosity ratio should be slightly less than one. This would lead touniform interfaces between layers in coextrusion applications and it islikely that the same behavior holds true for coinjection. Having bothratios significantly less than one or both significantly more than onemay lead to problems with layer uniformity.

[0058] The melt viscosity and elasticity of polyesters may be altered bymodifying the polymer compositions, lowering the I.V. of the polyester,and/or by the careful selection of processing conditions for bothpolymers. Thus, those skilled in the art could readily produce polymershaving the desired viscosity and elasticity ratios using the process ofthe present invention. For example, a CHDM-modified PET with adramatically reduced I.V. can be coinjected with EVOH under appropriateprocessing conditions in such a manner that preforms and the resultingbottles can be produced without visual defects such as chevrons or otherflow anomalies.

[0059] Estimation of the Viscosity and Elasticity (or Relaxation Time)

[0060] Estimation of the viscoelastic parameters for a resin requiresthe appropriate Theological test data. In the present invention,frequency sweeps on a cone and plate rheometer are used to obtaindynamic viscosity information on the polymer melt. This test, which iswell known in the art, provides a complex viscosity η*, a storagemodulus G′ and a loss modulus G″, all as a function of the oscillationfrequency co.

[0061] An example set of data is shown in FIG. 1 for EVOH at 180° C. Forpurposes of the detailed description, the complex viscosity η* isapproximately the same as the steady shear viscosity η. Similarly, theoscillation frequency is approximately the same as the shear rate for asteady shear test. The storage modulus G′ is a direct measure of the“rubberiness” of the polymer, whereas G″ is related to the amount ofviscous dissipation (similar to the viscosity).

[0062] For coextrusion/coinjection optimization, the relaxation time λand the zero shear viscosity η_(o) are extracted. One method ofextraction is to fit all of the data to one of many constitutiveequations available in the literature. However, an easier method is toestimate the parameters graphically. The zero shear viscosity η_(o) canbe estimated by extrapolating η* to very small values of ω (this has avalue of 93020 poise in FIG. 1). For purposes of this description, wewill refer to the fitted value of η_(o) as simply η, although it shouldbe understood by the reader that the true steady shear viscosity η isreally frequency (or shear rate) dependent. To estimate λ, the frequencyω* where G′ and G″ intersect must be found. The relaxation time λ canthen be approximated as 1/ω* where, for this EVOH example, we get avalue of 0.006 S.

[0063] Because both λ and η vary with temperature, it is important torepeat this dynamic viscosity measurement at different temperatures. Atleast 3 sweeps for each resin are usually performed. The parameters ηand λ can both be curve fit to an Arrhenius type of equation withactivation energy E_(a) having the form:

[0064] η=A exp(Ea/RT)

[0065] λ=B exp(Ea/RT)

[0066] where A and B are front factors, T is temperature and R is thegas constant. Once values of A, B, and Ea/R are fitted, the viscosityratio and elasticity ratio for any two polymers at any given set of melttemperatures can be calculated.

[0067] Factors for Forming Coinjected or Coextruded Articles

[0068] Some factors to consider when forming a multilayer article thathas minimal interfacial instabilities (i.e., chevrons, wavy lines, etc.)according to this invention include, but are not limited to thefollowing:

[0069] 1. Interfacial instabilities occur when the shear stress at theinterface between two layers exceeds a certain critical value (thisvalue depends on the resins involved). Thus, by keeping the stress atthe interface to a minimum during coinjection or coextrusion, theinstabilities can be eliminated.

[0070] 2. Shear stresses increase as the shear rate increases. Becauseof the shape of the velocity profile in the flow channel, the shear ratetends to be a maximum near the wall (this can be the die wall in acoextruded structure or the mold wall in coinjection) and zero near thecenter of the flow channel (see FIG. 2). As a result, shear stresses arehighest near the wall and lowest at the center. Thus, the closer theinterface is to the wall surface, the more likely the interfacial stresswill exceed the critical stress such that visual defects form.Interfaces near the center of the flow channel are unlikely to developflow instabilities because the stress is low. Thus, where theapplication allows (not all do), it is preferred to have the interfaceas close to the center as possible.

[0071] 3. Shear stresses are also increased when the outermost layer isat a higher viscosity than the next innermost layer. Therefore, bymaintaining viscosities where the outer layer is at a lower viscositythan an inner layer, the stresses are minimized and the instabilitiesare minimized. The closer the interface is to the mold wall, the lowerthe viscosity of the more outer layer should be.

[0072] 4. Shear stresses can also be reduced by lowering the flow rateof polymer through the die (or the rate at which it is injected into themold). However, lowering the flow rate implies a reduction in linespeed, which is not economically attractive.

[0073] 5. Coextrusion often involves having the “performance” layer as athin cap layer on the outside of the sheet (e.g., for UV block, highergloss, heat sealing, etc.). As a general rule, whenever this cap layeris less than about 10% of the total sheet thickness, the interfacialstress is likely to be high enough to cause instabilities. Thin caplayers are also believed to cause layer “wavy line” oscillations thatstart in the feedblock as opposed to the die. They occur when the angleof impingement of the flow channels is too high. They can usually bealleviated by bringing the layers together more gradually (i.e., asmaller impingement angle) and by keeping the outer layer viscosity lowjust as with the regular flow instabilities. Thus methods for reducingregular flow instabilities described herein also help to eliminate thewavy lines.

[0074] 6. During coextrusion, the interfacial instabilities are mostlikely to start to form in the die land region just before reaching thedie lips. This is because the flow channel is narrowest there and thestresses are higher. In contrast, the wavy lines mentioned previouslyusually start at the impingement point in the feedblock or coinjectionchannel.

[0075] 7. Unlike coextrusion, coinjection rarely involves that the“performance layer” be added as a thin cap layer on the outer edge.Thus, it would seem that the problems of instabilities should be minor.However, coinjection involves much higher shear rates than coextrusion,so the interfacial stresses can be significantly high, even far removedfrom the mold wall. In addition, the polymer is being rapidly cooledfrom the wall surface inward, which effectively narrows the flow channelas polymer solidifies and thus raises the stresses even further. Asmentioned before, the closer the coinjected barrier layer is to thecenterline, the less likely that instabilities will form.

[0076] Detailed Description of the Embodiments

[0077] 1. Coinjection

[0078] In one embodiment for forming a multilayer article by acoinjection process according to this invention, the ratio of viscosityfor an outermost layer (A), which is closest to the mold wall andtypically called a “cap” layer, over the next innermost layer (B), whichis closer to the center and typically called a “core” layer, should beless than or equal to (#) about 2. In other words, η(A)/η(B) #2 where ηis viscosity, η(A) is the outermost layer viscosity and η(B) is the nextinnermost layer viscosity. A more preferred embodiment is whereη(A)/η(B) is greater than or equal to about 0.5 and less than or equalto about 1 (0.5 #η(A)/η(B) #1).

[0079] For most, but not all coinjection applications, the structuralresin (e.g., PET) will be resin A (the outermost layer) and resin B willbe the inner barrier layer since it is near the center. In a similarmanner to coextrusion, if the barrier layer is in the center of thewall, then η(A)/η(B) is preferred to be closer to 1. As the barrierlayer location moves closer to the wall, then η(A)/η(B) should getsmaller, preferably approaching 0.5.

[0080] However, other numerous multilayer embodiments are contemplatedby this invention. For example, a multilayer article of this inventionmay be prepared from a coinjection of 5 layers where (starting from oneside) the layers are arranged as follows: PET/EVOH/PET regrind/EVOH/PET.In this example, the (PET)/η(EVOH) #2 and η(EVOH)/η(PET regrind) #2.Preferably, each ratio is from 0.5 to 1.

[0081] Thus, to form the above 5 layered article, η(PET) #η(EVOH) #η(PETregrind). Unfortunately, this is not always easy since the regrind PETis usually at a lower I.V. (and thus lower viscosity) than the regularPET. Nevertheless, this is the optimum condition that will give the bestpreform (or coextruded film) with no chevrons and/or instabilities.Further, since the viscosities are temperature dependent in that theviscosity decreases with increasing temperature, the various processingtemperatures to help achieve the conditions above may be changed (e.g.,PET regrind may run colder to increase its viscosity).

[0082] It must also noted that having the viscosity ratio of η(A)/η(B)of less than or equal to 2, more preferably of less than or equal toabout 1, helps reduce pumping pressures because the less viscous fluidnear the wall is serving as a lubricant.

[0083] In another embodiment, a multilayer article of this invention maybe prepared from a coinjection of 5 layers where (starting from oneside) the layers are arranged as follows: PET/MXD6/PET regrind/MXD6/PET.MXD6 is poly(m-xylylene adipamide) and acts as the performance layerwith barrier properties in the 5-layer structure. In this example, theη(PET)/η(MXD6) #2 and η(MXD6)/η(PET regrind) #2. Preferably, each ratiois from 0.5 to 1.

[0084] In yet another embodiment, a multilayer article of this inventionmay be prepared from a coinjection of 5 layers where (starting from oneside) the layers are arranged as follows: PET/PET regrind/MXD6/PETregrind/PET. In this example, the η(PET)/η(MXD6) #2 and η(MXD6)/η(PETregrind) #2. Preferably, each ratio is from 0.5 to 1. EVOH may also bethe barrier or performance layer in this embodiment thereby forming a5-layer article arranged as follows: PET/PET regrind/EVOH/PETregrind/PET. Again, the η(PET)/η(EVOH) #2 and η(EVOH)/η(PET regrind) #2.

[0085] Another embodiment involves coinjecting a multilayer article suchthat the viscoelastic properties of the resins (i.e., viscosity andelasticity) are properly balanced to achieve the best layer thicknessuniformity. In particular, the resins are chosen so that, at givenprocessing/melt temperatures, both the viscosity ratio η(A)/η(B) and therelaxation time ratio λ(A)/λ(B) are approximately less than or equal toabout 2 or otherwise balanced according to this invention. Mostpreferably, it is desired that the elasticity ratio is approximately thereciprocal of the viscosity ratio. If this condition is not met, thenlayer rearrangement will occur in the adapter and the thicknessdistribution will be altered.

[0086] An example of this is two polymers having different viscoelasticproperties. As the two resins flow together down a channel, resin Agradually wraps around and “encapsulates” resin B. The longer thechannel, the greater the degree of encapsulation. In coextrusion, thisencapsulation generally occurs in the adapter that connects thefeedblock to the die. Therefore it is important to keep the adapterlength short to minimize this encapsulation. Once the encapsulatedpolymer reaches the die, it fans out into the full sheet widthessentially “locking in” whatever distorted shape was present at the endof the adapter.

[0087] To optimize the conditions to eliminate both the visual defectsand the poor layer distribution, it is usually (but not always)important that both sets of conditions are simultaneously met. These“operating windows” are probably easier understood when showngraphically. FIG. 3 plots the relaxation time ratio of λ_(cap)/λ_(core)versus the viscosity ratio of η_(cap)/η_(core) where the “cap” resindenotes the outermost cap layer “A” and the “core” resin denotes theinnermost core layer “B.” For any given resins and processingtemperatures, this will produce an “operating point” somewhere on thegraph. For this operating point to satisfy the instability criterion ofη(A)/η(B) #2 and more preferably 0.5 #η(A)/η(B) #1, it should fall tothe left of the “y-axis” as denoted by the shaded area. For the flow tomaintain layer uniformity, it is most preferable that the conditionsfall approximately along a 45-degree line (from the upper left-handcorner to the lower right hand corner). The shaded ellipse in FIG. 3depicts this region. The closer the operating point to this diagonalline, the more uniform the layer structure. As the operating point movesfurther away in either direction, the layer distribution becomes poorer,as depicted in the diagram.

[0088] If both uniform layer distribution and elimination of visualdefects are to be obtained simultaneously, then the operating point mustfall in the upper left hand quadrant along the 45 degree line (where thetwo operating regimes intersect). This is therefore the true optimumprocessing point for most coextrusion and/or coinjecting applications.

[0089] For coinjection applications, it is still desirable to meet thesesame criteria although the reciprocal balancing of the viscosity andelasticity ratios is for a slightly different reason. In coinjection,the problem of poor layer distribution across the width does not existsince it is a symmetrical annular flow pattern. However, as describedearlier, by balancing the viscoelastic flow properties, we can help tominimize the chance of layer distortion as it flows around any bends orelbows in the connecting piping and gate.

[0090] 2. Coextrusion

[0091] There are four items preferred for a coextruded article orstructure:

[0092] (1) Good layer thickness distribution across the width of thesheet;

[0093] (2) The absence of interfacial flow instabilities (e.g. wavylines, chevrons, etc.);

[0094] (3) Good layer adhesion; and

[0095] (4) Minimal curling/warping of the final film/sheet.

[0096] The method presented herein only addresses the first two items asgood adhesion is more a matter of the chemistry differences between two,three or five resin layers. Poor adhesion is often corrected by a tielayer. Similarly, curling is related primarily to roll coolingconditions and is rarely a “fatal flaw”.

[0097] To understand where items (1) and (2) come into play requires anunderstanding of a typical coextrusion feedblock and die setup for aflat film coextrusion. Depending on the feedblock plate configuration,two or three resins could be brought together to form a 2-layer A/Bstructure, a 3-layer A/B/A structure or a 5-layer A/B/C/B/A orstructure. The 3-layer A/B/A structure, being symmetric (or “balanced”)about the center plane, is the easiest to make. Non-symmetric structuresare more prone to curling and warping due to the differences in thermalexpansion and relaxation during cooling. Fortunately, for the purposesof eliminating flow instabilities and balancing the layer thickness, itusually does not matter whether or not the structure is symmetric.

[0098] Many coextruded multilayer embodiments are contemplated by thisinvention. For example, a multilayer article of this invention may beprepared from coextruding 5 layers where (starting from one side) thelayers are arranged as follows: PET/EVOH/PET regrind/EVOH/PET. In thisexample, the η(PET)/η(EVOH) #2 and η(EVOH)/η(PET regrind) #2.Preferably, each ratio is from 0.5 to 1.

[0099] To form the above 5 layered article, η(PET) #η(EVOH) #η(PETregrind). Unfortunately, this is not always easy since the regrind PETis usually at a lower I.V. (and thus lower viscosity) than the regularPET. Nevertheless, this is the optimum condition that will give the bestcoextruded film with no chevrons and/or instabilities. Further, sincethe viscosities are temperature dependent in that the viscositydecreases with increasing temperature, the various processingtemperatures to help achieve the conditions above may be changed (e.g.,PET regrind may run colder to increase its viscosity).

[0100] In another embodiment, a multilayer article of this invention maybe prepared from coextruding 5 layers where (starting from one side) thelayers are arranged as follows: PET/MXD6/PET regrind/MXD6/PET. MXD6 ispoly(m-xylylene adipamide) and acts as the performance layer withbarrier properties in the 5-layer structure. In this example, theη(PET)/η(MXD6) #2 and η(MXD6)/η(PET regrind) #2. Preferably, each ratiois from 0.5 to 1.

[0101] In yet another embodiment, a multilayer article of this inventionmay be prepared from coextruding 5 layers where (starting from one side)the layers are arranged as follows: PET/PET regrind/MXD6/PETregrind/PET. In this example, the η(PET)/η(MXD6) #2 and η(MXD6)/η(PETregrind) #2. Preferably, each ratio is from 0.5 to 1. EVOH may also bethe barrier or performance layer in this embodiment thereby forming a5-layer article arranged as follows: PET/PET regrind/EVOH/PETregrind/PET. Again, the η(PET)/η(EVOH) #2 and η(EVOH)/η(PET regrind) #2.

[0102] Laver Rearrangement and Encapsulation

[0103] The different layers are brought together inside the feedblockalthough how they are brought together depends on the type of feedblock(e.g., Welex, Dow, Cloeren, etc). After impingement, the layers flowside by side through the adapter and into the die. The adapter can haveany of a number of different cross-sectional shapes although circularand rectangular are the most common. It is in the adapter where problemswith layer thickness uniformity usually arise. If the viscosity (or aswill be discussed later, the “elasticity”) of A is lower than B then itwill tend to wrap around or “encapsulate” B as it flows down theadapter. Similarly, if B has a lower viscosity than A, then it will tryto encapsulate A. To keep this encapsulation to a minimum, it is usuallyrecommended that the ratio of viscosities for A and B be kept less thanabout 2 (or greater than about 0.5). This general rule works in manyinstances but fails in many others. As will be discussed in the nextsection, this failure resulted because elasticity effects, which havepreviously been neglected, are as important than the viscosity effects.Thus, it becomes important to balance the elasticity and viscosityratios simultaneously.

[0104] Interfacial Instabilities

[0105] Whereas layer rearrangement occurs primarily in the adapterregion, interfacial instabilities occur primarily in the die where shearrates are higher (100 to 1000 s⁻¹ in the die versus 10 to 30 s⁻¹ in theadapter). When the shear stress at the interface between layers getsabove a certain critical value, flow instabilities occur. Theseinstabilities result in the wavy lines, chevrons, and ripples that areunacceptable for most end-use applications.

[0106] There are three main factors that contribute to high interfacialstresses and thus to flow instabilities. These are (a) thin cap layers(<10% total thickness), (b) cap layers having a viscosity which ishigher than the next innermost layer and (c) overall throughput ratesthat are too high. In other words, interfacial instabilities are mostlikely to occur when thin cap layers are present, particularly if theviscosity of the cap layer is higher than the core layer. This isbecause shear stresses tend to be higher near the outer wall of theadapter or flow channel. For interfaces near the center of the adapter(e.g., a 50/50 A/B structure), it is very rare for flow instabilities tooccur because the stresses are already low. Even for thin cap layers, ifthe cap layer viscosity is kept lower than the core layer, theninterfacial instabilities can usually be eliminated. This is importantsince thin cap layers are very common.

[0107] Finally, as noted in item (c) above, another simple way toeliminate flow instabilities is to reduce the overall throughput ratethrough the die. This will reduce the interfacial shear stress althoughit may result in economically unacceptable line speeds. Reduction inthroughput rate should only be used as a “tweaking” adjustment ifproblems arise on the line. It is better to properly select the resinsin the initial design phase so that higher throughput rates can bemaintained.

[0108] Elasticity in Coextrusion

[0109] Elasticity plays a role in coextrusion. Interestingly, the effectof elasticity on layer uniformity is probably more important thanviscosity, which may explain why using only viscosity ratios to predictflow behavior rarely works.

[0110] The first step in understanding the effects of elasticity is todefine exactly what is meant by “elasticity”. Elasticity is the rubberylike behavior of the fluid—the ability for the melt to have a memory. Onone extreme are materials that are purely viscous with no elasticity.Examples include water, glycerin, air, etc. On the other extreme arematerials that are purely elastic with no significant viscosity.Examples of purely elastic materials include rubber bands, most metals,and solids in general. Polymer melts fall somewhere in between, beingboth viscous and elastic at the same time (i.e., viscoelastic).

[0111] One of the easiest methods for quantifying the viscous andelastic portions of a polymer melt is to measure the dynamic viscosity(or dynamic modulus) using a cone and plate rheometer. This method iswell known to those skilled in the art and need not be described indetail herein. It is emphasized that all of these properties arefunctions of the effective shear rate.

[0112] As discussed above, elasticity and relaxation time aresynonymous. The elasticity ratio should preferably be from about 0.5 toabout 2 for coextrusion to typically be successful. Similarly, if thecap layer relaxation time is lower than the core layer, it will try toencapsulate the core (just as if the cap layer viscosity was lower thanthe core layer). Likewise, if the cap layer relaxation time is higherthan the core, then the core layer will try to encapsulate the caplayer. In effect, it is just as important to balance the elasticities asit is to balance the viscosities.

[0113] After defining the two key parameters for each resin, the resultsmay be combined and predictions about the flow may be made. It turns outthat interfacial instabilities and layer uniformity/rearrangement can betreated independently during the analysis. This is preferable sincethere are times when some interfacial instabilities are allowed (e.g.,in opaque sheet) although uniform layer distribution across the width ofthe sheet is critical. It is also emphasized that “good layerdistribution” really depends on the application. For most applications,it is desirable to have a constant thickness of each layer across theentire width of the sheet (this minimizes edge trim). However, there aresome applications where it is more desirable to have the cap layercompletely encapsulate the core (in an A/B/A structure), especially whenthe core layer may pose some sort of hazard (even on the outer edges).One example of this might be where post-consumer recycle is the corelayer (or some other non-FDA resin) in a food-contact application.

[0114] The steps for performing the coextrusion analysis for predictingflow are listed below:

[0115] 1. Test each resin for a dynamic viscosity sweep (cone and platerheometer) at 3 or more different reasonable temperatures (i.e., don notuse temperatures where the resin can not be processed or will degradeexcessively).

[0116] 2. Determine η_(o) and λ for each resin and at each temperature.

[0117] 3. Curve fit η_(o) and λ versus T using an Arrhenius plot foreach resin to determine activation energies and front factors. Oneskilled in the art would understand without description how to fit η_(O)and λ versus T in an Arrhenius plot and as such a detailed descriptionof this technique is not necessary. Although not required, this willmake extrapolation of the results to different temperatures easier in alater part of the analysis.

[0118] 4. Calculate the viscosity ratio and elasticity ratio(η_(oA)/η_(oB) and λ_(A)/λ_(B)) as a function of melt temperature foreach resin pair. By convention, the numerator of each resin representsthe “outermost” layer and is the one closest to the wall. For atwo-layer coextrusion structure (A/B) the outermost layer is usuallytaken to be the one that is thinnest.

[0119] 5. Determine whether interfacial instabilities are a problem fora given melt temperature. If the cap layer is thin (less than 10%) andη_(oA)/η_(oB) greater than or equal to 2, then interfacial instabilitieswill likely occur. It is most preferable, therefore, to keepη_(oA)/η_(oB) less than or equal to 1 to prevent these instabilities.

[0120] 6. Determine layer uniformity. If η_(oA)/η_(oB) and λ_(A)/λ_(B)are both greater than 1, then the core layer will encapsulate the cap.The degree increases the further away from 1 the ratios get. If both areless than one, then the cap will encapsulate the core. For uniform layerdistribution, both ratios should be close to 1. Also, if η_(oA)/η_(oB)is greater than or equal to about 1 and λ_(A)/λ_(B) is less than orequal to about 1 (or vice-versa), then the encapsulation effects willoffset and the layers will be roughly uniform.

[0121] Below is a discussion of some of the above steps in more detail.

[0122] Step 1: Dynamic Viscosity

[0123] Each polymer in the coextruded article or structure should betested via standard dynamic viscosity sweeps using a cone and plate (orparallel plate rheometer) (See FIG. 1). This is a standard test. Atleast three or more temperatures for each resin should be run althoughthese temperatures should represent “typical” extrusion temperatures.For example, for PET, temperatures of 260, 280 and 300EC can be used.Below 260EC, the polymer would not melt and above 300EC, degradationbecomes significant.

[0124] Step 2: Determination of η_(o) and λ for Each Resin

[0125] As described previously, values for η_(o) and λ should beextracted from the dynamic viscosity data for each temperature and foreach resin. The parameter η_(o) is the η* viscosity at low shear rates.The relaxation time λ is equal to l/w* where w* is the angular velocitywhere G′ and G″ intersect. For many resins, G′ and G″ will intersect “onthe page” and within the plot range for w*. For some resins, however,one will have to extrapolate G′ and G″ off of the page in order todetermine an intersection point.

[0126] Step 4: Calculation of the Viscosity and Elasticity Ratios

[0127] Once the values of η_(o) and λ are calculated for each resin andat different temperatures, it is possible to determine the viscosity andelasticity ratios as a function of melt temperature. It is often assumedthat the two resins are extruded at the same melt temperature. Even ifthey are melted and processed at different temperatures, they willusually equilibrate to some average temperature within the feedblock andadapter so the constant temperature assumption is reasonable. This isparticularly true if one layer is a very thin compared to other layers.This ratio calculation should repeated for each resin pair interface inthe film.

[0128] Step 5: Determination of the Onset of Interfacial Instabilities

[0129] Interfacial instabilities will usually only occur in thin caplayers (or in thin die layers if they are close to the outer edge) whenthe viscosity of the cap layer is higher than the core layer (elasticityis not a significant factor here). The general rule of thumb then isthat interfacial instabilities will occur when η_(oA)/η_(oB) is greaterthan or equal to 2. Thus, to prevent the instabilities, η_(oA)/η_(oB) ispreferably less than or equal to 2. How much lower than 2 really dependson the throughput rate and the thickness of the cap layer A. For verythin cap layers and/or high line speeds, η_(oA)/η_(oB) is less than orequal to 2 and should most preferably be less than about 1. Having a lowviscosity cap layer serves as a sort of lubricant which minimizespumping pressures as well as eliminating interfacial instabilities. Thedisadvantage of having a low viscosity cap layer is that it will tend toencapsulate the core resin.

[0130] Step 6: Determination of Layer Uniformity

[0131] Proper determination of layer uniformity across the width of thesheet requires knowledge of both the viscosity and elasticity ratios.Also the type of coextrusion where uniformity may be a problem is forflat film dies whereas annular dies (e.g., blown film, pipe, preformmolding) will not exhibit the same across the width variability.

[0132] If the calculations for the initial resins result in unacceptableoperating conditions, it is still possible to correct the problem. Anumber of ways to do this are described below. These techniques apply tocoinjection and coextrusion.

[0133] Changing the Molecular Weight (or I.V.)

[0134] The first, and most logical method is to change the molecularweight (or I.V.) of one of the resins. This is because η_(o) isproportional to M_(w) ^(3.4) (or η_(o) is proportional to I.V.^(5.1) ).The relaxation time λ also follows the same M_(w) (or I.V.) dependence.So, for example, by increasing the I.V./M_(w) of one of the resins, wechange both η_(oA)/η_(oB) and λ_(A)/λ_(B) in a similar manner. Thus, ifwe increase the cap layer M_(w), both η_(oA)/η_(oB) and λ_(A)/λ_(B) willincrease. Similarly, decreasing the cap layer M_(w) will cause theoperating point to decrease. Changing the core layer viscosity (resinB), has a similar effect although the directions are reversed. Often thechoice of whether to vary the cap or core layer molecular weight isrestricted by what resin formulations are commercially available.

[0135] Adding a Branching/Crosslinking Agent

[0136] Changing the M_(w), or I.V. of one of the resins causes bothη_(oA)/η_(oB) to change in the same direction. There are situationswhere this is not desirable and it is preferred to change the elasticityand viscosity ratios independently. Adding a brancher/crosslinkingprimarily affects the elasticity and is therefore a nice method forvarying λ_(A)/λ_(B) without significantly altering η_(oA)/η_(oB). Forpolyesters, this brancher might be a typical multifunctional branchingagent like trimellitic anhydride (TMA) or pyromellitic dianhydride(PMDA). For polyethylene, blending in LDPE (assuming either LLDPE orHDPE is being used) can increase the branching.

[0137] Changing the Melt Temperatures

[0138] Up until now, it has generally been assumed that the two resinsare at the same melt temperature. This is not an unreasonable assumptionsince some thermal equilibration will occur in the feedblock, adapterand die. Still it is possible to run the polymers at slightly differentmelt temperatures (usually 25EC is considered the maximum temperaturedifferential). Running a polymer at a slightly different temperature hasthe same effect as if the M_(w) had been changed. For example, if thecap layer is at a slightly hotter temperature, its elasticity andviscosity are reduced relative to the nominal melt temperature. Thisshifts the operating point along the same diagonal line associated withchanging M_(w)/I.V.

[0139] If different temperatures, are used, it is necessary to modify(5) slightly since viscosities and relaxation times must be extractedfor each polymer at the appropriate temperature. The simplest approachis to use (4), plugging in the appropriate temperatures for each resin,and then manually calculating the ratios. While changing melttemperatures has the same effect as changing I.V./M_(w), the effects arenot as significant. Therefore, varying the melt temperature should onlybe used as an online “tweaking” adjustment.

[0140] Changing the Feedblock Design

[0141] Cutting metal is always considered a last resort and is usuallyon applied when layer uniformity is unacceptable and no othermodification seems to work. Typically, as with a Welex block, a flowplate is altered so that when the resins impinge on one another there issome compensation for the encapsulation. For example, if the cap layeris encapsulating the core, the flow plate is modified so that when thelayers first impinge, the core layer is partially wrapped around the capby an equal amount. As the resins flow towards the die, the cap layerwill still try to flow around the core.

[0142] However, another approach to minimizing the amount ofencapsulation is to shorten the adapter length. A long adapter providesmore time for the resins to rearrange before reaching the die. TheCloeren multi-manifold die takes this approach to an extreme since thelayers are literally brought together inside the die with no realadapter to speak of. The Cloeren die is expensive but useful whenviscosity ratios (or elasticity ratios) are extreme. Layer rearrangementmay still want to occur, but is not given enough time to actuallyhappen.

[0143] 3. Structural Layer

[0144] In accordance with the present invention, and in a preferredembodiment, the structural layer comprises one or more polymers thatprovide the mechanical and physical properties required of a packagematerial or article. Suitable polymers comprise, but are not limited to,any polyester homopolymer or copolymers that are suitable for use inpackaging, and particularly food packaging. The more preferred polyesteris PET, including PET regrind.

[0145] Suitable polyesters useful in the present invention are generallyknown in the art and may be formed from aromatic dicarboxylic acids,esters of dicarboxylic acids, anhydrides of dicarboxylic esters,glycols, and mixtures thereof. Suitable partially aromatic polyestersare formed from repeat units comprising terephthalic acid, dimethylterephthalate, isophthalic acid, dimethyl isophthalate, dimethyl-2,6naphthalenedicarboxylate, 2,6-naphthalenedicarboxylic acid, 1,2-, 1,3-and 1,4phenylene dioxydoacetic acid, ethylene glycol, diethylene glycol,1,4-cyclohexane-dimethanol, 1,4-butanediol, and neopentyl glycolmixtures thereof.

[0146] Preferably, the structural polyesters comprise repeat unitscomprising terephthalic acid, dimethyl terephthalate, isophthalic acid,dimethyl isophthalate, and/or dimethyl-2,6-naphthalenedicarboxylate. Thedicarboxylic acid component of the polyester may optionally be modifiedwith one or more different dicarboxylic acids (preferably up to about 20mole %). Such additional dicarboxylic acids comprise aromaticdicarboxylic acids preferably having 8 to 14 carbon atoms, aliphaticdicarboxylic acids preferably having 4 to 12 carbon atoms, orcycloaliphatic dicarboxylic acids preferably having 8 to 12 carbonatoms. Examples of dicarboxylic acids to be comprised with terephthalicacid are: phthalic acid, isophthalic acid, naphthalene-2,6-dicarboxylicacid, cyclohexanedicarboxylic acid, cyclohexanediacetic acid,diphenyl-4,4′-dicarboxylic acid, succinic acid, glutaric acid, adipicacid, azelaic acid, sebacic acid, mixtures thereof and the like.

[0147] In addition, the glycol component may optionally be modified withone or more different diols other than ethylene glycol (preferably up toabout 20 mole %). Such additional diols comprise cycloaliphatic diolspreferably having 6 to 20 carbon atoms or aliphatic diols preferablyhaving 25 to 20 carbon atoms. Examples of such diols comprise:diethylene glycol, triethylene glycol, 1,4-cyclohexanedimethanol,propane-1,3-diol, butane-1,4-diol, pentane-1,5-diol, hexane-1,6-diol,3-methylpentanediol-(2,4), 2-methylpentanediol-(1,4),2,2,4-trimethylpentane-diol-(1,3), 2-ethylhexanediol-(1,3),2,2-diethylpropane-diol-(1,3), hexanediol-(1,3),1,4-di-(hydroxyethoxy)-benzene, 2,2-bis-(4-hydroxycyclohexyl)-propane,2,4-dihydroxy-1,1,3,3-tetramethyl-cyclobutane,2,2-bis-(3-hydroxyethoxyphenyl)-propane,2-bis-(4-hydroxypropoxyphenyl)-propane, hydroxyethyl resorcinol,mixtures thereof and the like. Polyesters may be prepared from two ormore of the above diols.

[0148] The resin may also contain small amounts of trifunctional ortetrafunctional comonomers such as trimellitic anhydride,trimethylolpropane, pyromellitic dianhydride, pentaerythritol, and otherpolyester forming polyacids or polyols generally known in the art.

[0149] 4. Performance Layer

[0150] At least one layer in a multilayer article of the presentinvention is a performance layer and provides the resulting article withimproved physical properties. Such properties comprise, but are notlimited to, barrier to migration (gas, vapor, and/or other smallmolecules), barrier to harmful light (ultraviolet light) and mechanicalproperties such as heat resistance.

[0151] In one embodiment, a multilayer article of the present inventionwhere the performance layer is a barrier layer displays improved CO₂and/or O₂ barrier compared to an article of unmodified PET homopolymer.In other embodiments, all of the layers are modified to display improvedproperties. Suitable materials for the barrier layers of the presentinvention comprise polyamides, ethylene-vinyl acetate copolymer (EVOH),polyalcohol ethers, wholly aromatic polyesters, resorcinol diaceticacid-based copolyesters, polyalcohol amines, isophthalate containingpolyesters, PEN and its copolymers and mixtures thereof. Barriermaterials may be used neat or may be modified to further improvebarrier, such as with the addition of platelet particles, preferablylayered clay material, such as those available from Nanocor, SouthernClay Products, Rheox and others.

[0152] Suitable polyamides comprise partially aromatic polyamides,aliphatic polyamides, wholly aromatic polyamides and mixtures thereof.By “partially aromatic polyamide,” it is meant that the amide linkage ofthe partially aromatic polyamide contains at least one aromatic ring anda nonaromatic species.

[0153] Suitable polyamides preferably have a film-forming molecularweight. Wholly aromatic polyamides preferably comprise, in the moleculechain, at least 70 mole % of structural units derived from m-xylylenediamine or a xylylene diamine mixture comprising m-xylylene diamine andup to 30% of p-xylylene diamine and an aliphatic dicarboxylic acidhaving 6 to 10 carbon atoms. These wholly aromatic polyamides arefurther described in Japanese Patent Publication Nos. 1156/75, 5751/75,5735/75 and No. 10196/75, and Japanese Patent Application Laid-OpenSpecification No. 29697/75.

[0154] Polyamides formed from isophthalic acid, terephthalic acid,cyclohexanedicarboxylic acid, meta- or para-xylylene diamine, 1,3- or1,4-cyclohexane(bis)methylamine, aliphatic diacids with 6 to 12 carbonatoms, aliphatic amino acids or lactams with 6 to 12 carbon atoms,aliphatic diamines with 4 to 12 carbon atoms, and other generally knownpolyamide forming diacids and diamines can be used. The low molecularweight polyamides may also contain small amounts of trifunctional ortetrafunctional comonomers such as trimellitic anhydride, pyromelliticdianhydride, or other polyamide-forming polyacids and polyamines knownin the art.

[0155] Preferred partially aromatic polyamides comprise: poly(m-xylyleneadipamide), poly(hexamethylene isophthalamide), poly(hexamethyleneadipamide-co-isophthalamide), poly(hexamethyleneadipamide-co-terephthalamide), and poly(hexamethyleneisophthalamide-co-terephthalamide). The most preferred partiallyaromatic polyamide is poly(m-xylylene adipamide).

[0156] Preferred aliphatic polyamides comprise poly(hexamethyleneadipamide) and poly(caprolactam). The most preferred aliphatic polyamideis poly(hexamethylene adipamide). Partially aromatic polyamides arepreferred over the aliphatic polyamides where good thermal propertiesare crucial.

[0157] Preferred aliphatic polyamides comprise polycapramide (nylon 6),poly-aminoheptanoic acid (nylon 7), poly-aminonanoic acid (nylon 9),polyundecane-amide (nylon 11), polyarylactam (nylon 12),polyethylene-adipamide (nylon 2,6), polytetramethylene-adipamide (nylon4,6), polyhexamethylene-adipamide (nylon 6,6),polyhexamethylene-sebacamide (nylon 6,10), polyhexamethylene-dodecamide(nylon 6,12), polyoctamethylene-adipamide (nylon 8,6),polydecamethylene-adipamide (nylon 10,6), polydodecamethylene-adipamide(nylon 12,6) and polydodecamethylene-sebacamide (nylon 12,8).

[0158] Suitable polyalcohol ethers comprise the phenoxy resin derivedfrom reaction of hydroquinone and epichlorohydrin as described in U.S.Pat. No. 4,267,301 and U.S. Pat. No. 4,383,101. These materials can alsocontain resorcinol units and may in fact be all resorcinol units asopposed to hydroquinone units for the aromatic residue.

[0159] Suitable wholly aromatic polyesters (frequently called LCPs) areformed from repeat units comprising terephthalic acid, isophthalic acid,dimethyl-2,6-naphthalenedicarboxylate, 2,6-naphthalenedicarboxylic acid,hydroquinone, resorcinol, biphenol, bisphenol A, hydroxybenzoic acid,hydroxynaphthoic acid and the like.

[0160] Diacetic resorcinol copolymers are described in U.S. Pat. No.4,440,922 and U.S. Pat. No. 4,552,948 and consist of copolyesters ofterephthalic acid, ethylene glycol and a modifying diacid from 5 to 100mol % in the composition replacing terephthalate units. The modifyingdiacid is either m-phenylenoxydiacetic acid or p-phenylenoxydiacetic.Either one of these diacids can be employed either by themselves or asmixtures in preparation of copolyesters for this invention.

[0161] Suitable polyalcohol amines comprise those derived from reactionof either resorcinol bisglycidyl ether with an alkanol amine, such asethanolamine, or hydroquinone bisglycidyl ether with an alkanol amine.Mixtures of these bisglycidyl ethers can obviously also be used inpreparation of a copolymer.

[0162] Suitable isophthalate-containing polyesters comprise polyesterscomprising repeat units derived from at least one carboxylic acidcomprising isophthalic acid (preferably at least 10 mole %) and at leastone glycol comprising ethylene glycol.

[0163] Suitable poly(ethylene naphthalate) (PEN) and PEN copolymerscomprise polyesters comprising repeat units derived from at least onecarboxylic acid comprising naphthalene dicarboxylic acid (preferably atleast 10 mole %) and at least one glycol comprising ethylene glycol.

[0164] The most preferred performance layer for barrier is a saponifiedethylene-vinyl acetate copolymer (EVOH). The saponified ethylene-vinylacetate copolymer is a polymer prepared by saponifying an ethylene-vinylacetate copolymer having an ethylene content of 15 to 60 mole % up to adegree of saponification of 90 to 100%. The EVOH copolymer should have amolecular weight sufficient for film formation, and a viscosity ofgenerally at least 0.01 dL/g, especially at least 0.05 dL/g, whenmeasured at 300° C. in a phenol/water solvent (85 wt. %:15 wt. %).

[0165] Conventional processes all of which are well known in the art,and need not be described here can make the polymers of the presentinvention.

[0166] Although not required, additives normally used in polymers may beused, if desired. Such additives comprise colorants, pigments, carbonblack, glass fibers, impact modifiers, antioxidants, surface lubricants,denesting agents, UV light absorbing agents, metal deactivators,fillers, nucleating agents, stabilizers, flame retardants, reheat aids,crystallization aids, acetaldehyde reducing compounds, recycling releaseaids, oxygen scavenging materials, or mixtures thereof, and the like.

[0167] All of these additives and many others and their use are known inthe art and do not require extensive discussion. Therefore, only alimited number will be referred to, it being understood that any ofthese compounds can be used in any combination of the layers so long asthey do not hinder the present invention from accomplishing its objects.

[0168] Shaped, multilayer articles according to this invention comprisefilm, sheet, tubing, pipe, profiles, preforms and containers such asbottles, trays, cups and the like.

EXAMPLES

[0169] The following examples and experimental results are comprised toprovide those of ordinary skill in the art with a complete disclosureand description of particular manners in which the present invention canbe practiced and evaluated, and are intended to be purely exemplary ofthe invention and are not intended to limit the scope of what theinventors regard as their invention. Efforts have been made to ensureaccuracy with respect to numbers (e.g., amounts, temperature, etc.);however, some errors and deviations may have occurred. Unless indicatedotherwise, parts are parts by weight, temperature is in ° C. or is atambient temperature, and pressure is at or near atmospheric.

Example 1

[0170] Coinjection of Various Grades of PET with EVOH as the BarrierLayer

[0171] Three different grades of PET were evaluated for use in acoinjection trial with EVALCA EVOH grade F-101. The PET resins aretabulated below in Table 1. Each has a different I.V. and the level ofCHDM copolymer modification varied slightly between them as shown. Thehigher the I.V. of the resin, the greater the viscosity and elasticity(the copolymer modification has only a minor effect). TABLE 1 PET SampleI.V. (dL/g) Copolymer Modification Level #1 0.71 Moderate #2 0.77 High#3 0.80 High

[0172] A 5-layer structure was coinjected consisting ofPET/EVOH/PET/EVOH/PET. The EVOH layer was relatively close to theoutside of the structure (i.e., near the wall) so interfacial stressesare expected to be high. Barrel temperatures for the PET samples werenominally 285EC whereas the barrel temperatures for the EVOH were 185EC.However, heat transfer calculations show that because the EVOH layer isso thin, and is surrounded by the hotter PET, that it quickly reaches atemperature of approximately 170EC. This temperature was used toestimate η and λ. The operating points for each of these three PETresins with PET were determined according to this invention. Only Resin#1 is predicted to have both good uniform layer distribution and noinstabilities/chevrons.

[0173] An injection molding trial was held using samples #1 and #2.Previous attempts with sample #3 had already shown that it would notwork without causing chevrons. Under identical molding conditions, partsmolded using sample #1 were free of chevrons whereas the parts moldedwith sample #2 had a visible flow defects in the form of chevrons.

Example 2

[0174] Determination of an Optimum Coextrusion Window for anCap/Core/Cap Layer Coextrusion of polyethylene onto PETG 6763

[0175] A thin layer of polyethylene (Eastman Chemical Company CM 27057-F(2.0 MI)) was coextruded onto both sides of a PETG 6763 sheet. PETG 6763is a copolyester commonly used in the film and sheeting industry and hasan IV=0.76 dl/g (as measured in 60 wt. %/40 wt. %phenol/tetrachloroethane at 25° C.). The total thickness wasapproximately 40 mils with the polyethylene cap layers being 10% of thethickness each. The film width was approximately 20 inches. The factthat the cap layers are thin makes interfacial instabilities a distinctpossibility. The polyethylene (PE) had a melt index of 2.0. Dynamicviscosity measurements were performed at 220, 240 and 260° C. for thePETG and 230, 250 and 270° C. for the polyethylene and the values for ηand λ extracted at each temperature. These values were plotted alongwith the Arrhenius curve fit. Based upon the plot, the viscosity of thePETG is higher at lower temperatures but becomes lower above about 235°C. Because both of these resins can be processed over a wide range ofmelt temperatures (from about 200° C. to 300° C.), it was desired tofind the best set of temperatures to optimize the process.

[0176] To test the method for optimizing conditions, a designedexperiment was performed around the PE and PETG melt temperatures. Therun conditions were as follows in Table 2: TABLE 2 Run # T(Polyethylene)T(PETG) 1 210° C. 210° C. 2 210° C. 260° C. 3 260° C. 260° C. 4 260° C.210° C. 5 235° C. 235° C.

[0177] This provided a spread of temperatures covering a range ofpossible processing conditions. The viscosity ratio and elasticity ratio(PE over PETG since PE is the outermost layer) was determined for eachof the runs above according to the method outlined in the detaileddescription (See FIG. 4). Runs 1 and 5 will probably give the best layeruniformity since they are closest to the layer thickness optimum. As onemoves to the lower left in FIG. 4 (e.g. run 4), it is predicted that thethickness of the cap layer will get thicker near the edges and thinnernear the middle. As one moves to the upper right hand corner of FIG. 4(e.g. run 2), just the opposite is predicted to occur.

[0178] Because the PE has poor adhesion to the PETG, it was possible topeel apart the layers and measure the thickness across the width. Thecap layer thickness was found to go from being thick on the edges tothick in the middle as one moves from the lower left hand corner to theupper right hand corner in FIG. 4. Similarly, there is a crossover inthe thickness distribution at the same location that the optimum processcondition is predicted (between run 1 and run 5). Based on the model, itis predicted that the optimum run conditions be when both extruders areset at about 220EC. This prediction is verified by the experimentaldata.

Example 3

[0179] Determination of the Optimum Grade of Polyethylene to beCoextruded with PETG at 235EC

[0180] In this example, the extrusion temperature was fixed at 235EC forboth the PE and the PETG. Otherwise, the coextrusion equipment andconditions were the same as Example 2. Three different PE's wereevaluated including Eastman Chemical Company PE's: CM-27053-F (0.9 MI),CM-27057-F (2.0 MI) and CM-27058-F (3.2 MI). The higher the melt index(MI), the lower the molecular weight of the resin. This lowering of themolecular weight also correspondingly causes a decrease in both theviscosity and elasticity.

[0181] The operating points for these three resins coextruded onto PETGat 235EC are shown in FIG. 5. In addition, a small insert graph at eachpoint shows the thickness distribution of the PE cap layer for eachvalue of MI. As with changing the melt temperature in Example 2,changing the MI for the PE from high to low causes the layerdistribution of the cap layer to go from being heavy on the outer edges,to heavy in the middle. The optimum value of MI for uniform layerdistribution is predicted to be around 2.2 to 2.4. Nevertheless, the 2.0melt index sample, which was closest to the optimum area, was also thebest looking sample. In addition, the 0.9 MI sample had small chevronspresent as predicted by the model.

Example 4

[0182] Coextrusion of Eastman PET 9921 with a 5% Cap Layer of MAKROLON2608 Polycarbonate

[0183] A multilayer structure of PET with a thin cap layer ofpolycarbonate (PC) was coextruded. The polycarbonate added surface glossand also helps to stiffen the polymer since it softens at a highertemperature (the glass transition temperature of PET is 80EC and forpolycarbonate it is 150EC. Because the cap layer is thin, the formationof interfacial instabilities is a significant problem.

[0184]FIG. 6 shows the operating points for different extrusiontemperatures (the PET and PC temperatures were set the same). Below aprocessing temperature of 275EC, instabilities are predicted to occur.Extrusion trials on a 2.5″ extruder with a 24″ film die confirmed this.The processing temperature had to be set at 290EC or higher in order toeliminate the chevrons. This corresponds on the plot to a viscosityratio around 0.75 as would be expected for a relatively thin cap layer.Layer uniformity was generally good as predicted by the model.

Example 5

[0185] Five Layer Coextrusion of PET and MXD6

[0186] A 5 layer A/B/C/B/A coextrusion test was setup using smallKillion laboratory scale extruders to simulate what occurs in afive-layer coinjection process and also determine where best to placethe various layers. The PET resins used in the experiment were EastmanPET 9921 (0.80 IV) and Eastman PET 20007 (0.72 IV) and the nylon wasMXD6 6007. The Eastman PET 20007 was selected to represent an IV typicalof regrind material. It is the lowest viscosity resin of the threeresins at 280° C. Similarly, the MXD6 6007 has the highest viscosity ofthe three resins at 280° C. Therefore, the ideal structure to minimizechevrons based on the viscosity criterion would be to have the EastmanPET 20007 as the outer (A) layer, the PET 9921 as the intermediate (B)layer and the MXD6 as the core (C) layer.

[0187] As part of the experiment and to test this hypothesis, thevarious resins were placed in the A, B, and C layers in various, but notexhaustive combinations as shown in Table 3 below. The B and C extruderswere 1″ extruders although their maximum outputs were different (C had amaximum RPM of 57 whereas B had a maximum RPM of 107). The A extruderwas a 1.25″ extruder. All extruders, the feedblock and die were set at280° C. The screw RPM for each extruder, which roughly correlates withthe throughput rate, is also shown in Table 3. Chevrons occurred whenthe A cap layer thickness was very small (<10%) and only runs in thisthickness range are shown below.

[0188] The best films (runs 10 and 11) occurred when the higherviscosity MXD6 was at or near the center of the film. The worstchevron-filled films occurred when the MXD6 was the outer cap layer(layer A). This is in agreement with the predictions of the viscositycriterion. It further suggests that a 5 layer coinjected preformconsisting of PET/regrind/MXD6/regrind/PET might be more stable than thecurrent PET/MXD6/regrind/MXD6/PET structure, particularly if a lower IVPET is used for the structural resin (compare, for example, runs 11 and9). TABLE 3 Run# Layer A Layer B Layer C Summary of Film Quality 6 MXD620007 20007 chevrons on the surface (12 rpm) (100 rpm) (57 rpm) towardsthe middle of the film  7 MXD6 9921 20007 mild chevrons but larger (12rpm) (100 rpm) (57 rpm) than in run 6  8 MXD6 9921 9921 severe chevrons(12 rpm) (100 rpm) (57 rpm) 7 repeat MXD6 9921 20007 chevrons a littlebit (12 rpm) (100 rpm) (57 rpm) worse than in original run 7  9 20007MXD6 20007 chevrons formed and (15 rpm) (100 rpm) (57 rpm) disappearedin a periodic fashion 10 20007 MXD6 MXD6 minimal if any chevrons (15rpm) (100 rpm) (57 rpm) 11 20007 20007 MXD6 good film, no chevrons (15rpm) (100 rpm) (57 rpm) 9 repeat 20007 MXD6 20007 fairly clear with few(15 rpm) (100 rpm) (57 rpm) chevrons

[0189] Throughout this application, various publications are referenced.The disclosures of these publications in their entireties are herebyincorporated by reference into this application in order to more fullydescribe the state of the art to which this invention pertains.

[0190] It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the scope or spirit of the invention. Otherembodiments of the invention will be apparent to those skilled in theart from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims.

What is claimed is:
 1. A process for coextruding a multilayer articlecomprising coextruding at a selected coextruding temperature: (i) afirst outer polymer resin layer comprising at least one outer layerpolymer resin having at the selected coextruding temperature: a) aviscosity; b) a melting temperature; and c) a degradation temperature;and (ii) a second inner polymer resin layer comprising at least oneinner layer polymer resin having at the selected coextrudingtemperature: a) a viscosity; b) a melting temperature; and c) adegradation temperature; wherein the ratio of the outer layer polymerresin viscosity to the inner layer polymer resin viscosity at thecoextruding temperature is less than or equal to about 1, and whereinthe coextruding temperature is above the melting temperature of thehighest melting resin and below the degradation temperature of thelowest degrading resin to form a multilayer article.
 2. The process ofclaim 1, wherein component (i) has an elasticity, component (ii) has anelasticity, and the ratio of the component (i) elasticity to thecomponent (ii) elasticity at the coextruding temperature isapproximately the reciprocal of the viscosity ratio.
 3. The process ofclaim 1, wherein component (i) comprises at least one performancepolymer resin having an elasticity at the selected coextrudingtemperature.
 4. The process of claim 1, wherein component (ii) comprisesat least one structural polymer resin having an elasticity at theselected coextruding temperature.
 5. The process of claim 1, whereincomponent (ii) comprises at least one performance polymer resin havingan elasticity at the selected coextruding temperature.
 6. The process ofclaim 1, wherein component (i) comprises at least one structural polymerresin having an elasticity at the selected coextruding temperature. 7.The process of claim 1, wherein component (i) or (ii) is a barrierresin.
 8. The process of claim 1, wherein component (i) or (ii)comprises a polyamide or a copolymer thereof, an ethylene-vinyl acetatecopolymer (EVOH), a polyalcohol ether, a wholly aromatic polyester, aresorcinol diacetic acid-based copolyester, a polyalcohol amine, anisophthalate-containing polyester, poly(ethylene naphthalate) or acopolymer thereof, or a mixture thereof.
 9. The process of claim 8,wherein the polyamide comprises a partially aromatic polyamide, analiphatic polyamide, a wholly aromatic polyamide, or a mixture thereof.10. The process of claim 1, wherein component (i) or (ii) comprises asaponified ethylene-vinyl acetate copolymer (EVOH).
 11. The process ofclaim 1, wherein component (i) or (ii) comprises a polyester.
 12. Theprocess of claim 1, wherein component (ii) comprises a polyester. 13.The process of claim 12, wherein the polyester is a homopolymer or acopolymer.
 14. The process of claim 1, wherein component (i) or (ii)comprises an aromatic polyester comprising a repeat unit of terephthalicacid, dimethyl terephthalate, isophthalic acid, dimethyl isophthalate,dimethyl-2,6 naphthalenedicarboxylate, 2,6-naphthalenedicarboxylic acid,1,2-, 1,3- and 1,4-phenylene dioxydoacetic acid, ethylene glycol,diethylene glycol, 1,4-cyclohexanedimethanol (1,4-CHDM), 1,4-butanediol,neopentyl glycol or a mixture thereof.
 15. The process of claim 1,wherein component (i) or (ii) comprises poly(ethylene terephthalate) ora copolymer thereof.
 16. The process of claim 2, wherein the ratio ofthe outer polymer resin viscosity to the inner polymer resin viscosityat the coextruding temperature and the ratio of the outer polymer resinelasticity to the inner polymer resin elasticity at the coextrudingtemperature are approximately
 1. 17. The process of claim 2, wherein theratio of the outer polymer resin viscosity to the inner polymer resinviscosity at the coextruding temperature is less than or equal to about1 and greater than or equal to about 0.5 and the ratio of the outerpolymer resin elasticity to the inner polymer resin elasticity isapproximately the reciprocal of the viscosity ratio.
 18. The process ofclaim 1, wherein component (i) comprises an ethylene-vinyl acetatecopolymer (EVOH), component (ii) comprises poly(ethylene terephthalate)or a copolymer thereof, and the ratio of the outer polymer resinviscosity to the inner polymer resin viscosity and the ratio of theouter polymer resin elasticity to the inner polymer resin elasticity atthe coextruding temperature is approximately
 1. 19. A multilayer articleproduced by the process of claim
 1. 20. The article of claim 19 in theform of a film, sheet, tube, pipe, profile, preform, or container.
 21. Aprocess for coextruding a 5-layer article comprising coextruding at aselected coextruding temperature: (i) two outer polymer resin layerseach comprising an outer layer polymer resin having at the selectedcoextruding temperature: a) a viscosity; b) a melting temperature; andc) a degradation temperature; (ii) two intermediate polymer resin layersdisposed between a core layer and the two outer layers, the twointermediate resin layers each comprising an intermediate layer polymerresin having at the selected coextruding temperature: a) a viscosity; b)a melting temperature; and c) a degradation temperature; and (iii) acore layer comprising a core layer polymer resin having at the selectedcoextruding temperature: a) a viscosity; b) a melting temperature; andc) a degradation temperature; wherein at each polymer resin interfacethe ratio of the outermost polymer resin viscosity to the next innermostpolymer resin viscosity at the coextruding temperature is less than orequal to about 1 and the coextruding temperature is above the meltingtemperature of the highest melting resin and below the degradationtemperature of the lowest degrading resin to form a 5-layer article. 22.The process of claim 21, wherein at each interface of the five layersthe ratio of the outermost polymer resin viscosity to the next innermostpolymer resin viscosity at the coextruding temperature is less than orequal to about 1 and greater than or equal to about 0.5.
 23. The processof claim 21, wherein components (i), (ii) and (iii) have an elasticityat the coextruding temperature, and the ratio of the outermost polymerresin elasticity to the next innermost polymer resin elasticity at thecoextruding temperature is approximately the reciprocal of the viscosityratio.
 24. The process of claim 21, wherein component (i) comprises astructural polymer resin.
 25. The process of claim 21, wherein component(ii) comprises a structural polymer resin.
 26. The process of claim 21,wherein component (ii) comprises a performance polymer resin.
 27. Theprocess of claim 21, wherein component (iii) comprises a structuralpolymer resin.
 28. The process of claim 21, wherein component (iii)comprises a performance polymer resin.
 29. The process of claim 21,wherein component (i) comprises a structural polymer resin having astructural resin elasticity, component (ii) comprises a structuralpolymer resin having a structural resin elasticity, and component (iii)comprises at least one performance polymer resin having a performanceresin elasticity, and at each interface of the five layers the ratio ofthe outermost polymer resin elasticity to the next innermost polymerresin elasticity at the coextruding temperature is approximately thereciprocal of the viscosity ratio.
 30. The process of claim 21, whereincomponents (i) and (ii) comprise a polyester.
 31. The process of claim21, wherein components (i) and (ii) comprise an aromatic polyestercomprising repeat units of terephthalic acid, dimethyl terephthalate,isophthalic acid, dimethyl isophthalate, dimethyl-2,6naphthalenedicarboxylate, 2,6-naphthalenedicarboxylic acid, 1,2-, 1,3-or 1,4-phenylene dioxydoacetic acid, ethylene glycol, diethylene glycol,1,4-cyclohexanedimethanol (1,4-CHDM), 1,4-butanediol, neopentyl glycolor mixtures thereof.
 32. The process of claim 21 wherein component (i)comprises poly(ethylene terephthalate) or a copolymer thereof.
 33. Theprocess of claim 21, wherein component (ii) comprises poly(ethyleneterephthalate) regrind.
 34. The process of claim 21, wherein component(iii) is a barrier resin.
 35. The process of claim 21, wherein component(iii) comprises a polyamide or a copolymer thereof, an ethylene-vinylacetate copolymer (EVOH), a polyalcohol ether, a wholly aromaticpolyester, a resorcinol diacetic acid-based copolyester, a polyalcoholamine, an isophthalate-containing polyester, poly(ethylene naphthalate)or a copolymer thereof, or a mixture thereof.
 36. The process of claim35, wherein the polyamide comprises a partially aromatic polyamide, analiphatic polyamide, a wholly aromatic polyamide, or a mixture thereof.37. The process of claim 21, wherein component (iii) comprises asaponified ethylene-vinyl acetate copolymer (EVOH) or poly(m-xylyleneadipamide).
 38. The process of claim 21, wherein component (i) comprisespoly(ethylene terephthalate) or a copolymer thereof, component (ii)comprises poly(ethylene terephthalate) regrind and component (iii)comprises poly(m-xylylene adipamide), and at each polymer resininterface the ratio of the outermost polymer resin viscosity to the nextinnermost polymer resin viscosity at the coextruding temperature is lessthan or equal to about
 1. 39. A 5-layer article produced by the processof claim
 21. 40. The article of claim 39 in the form of a film, sheet,tube, pipe, profile, preform, or container.